Jeff Koons is back at Rockefeller Center. On the exact spot where his beloved Puppy (1992) sat in the Midtown Manhattan plaza 14 years ago, now a similarly flower-covered colossus depicting two toy heads halved and spliced, Split-Rocker (2000), stands 37 feet tall. The colorful, playful, and gently off-kilter sculpture’s installation, a Public Art Fund project, coincides with Koons’s career retrospective at the Whitney Museum, which opens June 27. While Puppy and Split-Rocker’s materials may be similar, Koons sees the newer piece as a very different beast.